Journal of Production Processes and Systems, vol. 6. (2012) No. 1., pp. 91-106. MAIN CHARACTERISTICS OF FUSION AND PRESSURE WELDING OF ALUMINIUM ALLOYS Imre Török 1, Krisztina Juhász 2, Ákos Meilinger 2, András Balogh 1 1 Associate Professor, 2 Research engineer University of Miskolc, Department of Mechanical Technology Abstract The aluminium and its alloys will be increasingly used for the welded lightweight structures. The essential condition of making high-quality welded joints is to know exactly the weldability and specifications of applied aluminium alloys. Modern, efficient processes should be applied for welding with good reproducibility. This paper provides an overview on the weldability of aluminium and its alloys and describes the modern fusion- and pressure welding processes as gas metal arc welding, tungsten inert gas, resistance spot welding and friction stir welding. Keywords: weldability of aluminium alloys, fusion welding, pressure welding, resistance spot welding, friction stir welding 1. Introduction Nowadays the utilization of aluminium and its alloys is increasing, more than that of the steels. Basically, this is owing to the application of aluminium alloys in automotive industry. More and more expansion of its application appears in the manufacturing of air transport, ship industry, on-shore (cars, trucks) and railway vehicles equally. The effect of high strength steels, titanium alloys and magnesium alloys of these areas mean a serious challenge in terms of its use. Aluminium alloy structures can only be competitive with the steel alloy structures if their weight loss and/or increase in service life can be achieved, additionally modern, reproducible and reliable joining technologies are used at the manufacturing of high-quality products. In the field of automotive industry the palette of aluminium and its alloys equally are in evidence hardenable, heat-treatable and cast alloys. These include pre-manufactured products as sheets, different profiles, moulded, pressed and forged products [1], [2]. Between the joint technologies of the pre-manufactured products of aluminium and its alloys appear almost all of the shape-, force- and material closing joints. Thus the welding appears on a preferred place at the manufacturing of structural elements [3]. In this condition we analyse the influence of important material properties at welding of aluminium and its alloys, presenting the most advanced, applied process variations of the welding processes, furthermore reveal research topics and results of the department in this field by few examples.
Imre Török, Krisztina Juhász, Ákos Meilinger, András Balogh 2. Material properties influencing the welding of aluminium alloys Some physical, chemical, mechanical and technological properties of aluminium and its alloys differ significantly from similar properties of other metals. These differences justify and determine the use of aluminium and its alloys, the application areas as well as significantly affect the weldability [2], [4]. Analysing the specialities and differences in terms of weldability of aluminium and its alloys should be noted in comparison with steel: relatively low melting point (480-660 C), there is no discoloration during fusion in contrast to steels, high affinity for oxygen, one of the highest among metals, the high melting point of surface coherent oxide layer (2050 C), high specific heat, twice as much steels, high thermal conductivity, three times greater than the same of steels, good electrical conductivity, four times of steels, high thermal expansion coefficient, twice of steels, varying hydrogen solvent ability, (in liquid-solid phase 20 to 1), different property of heat affected zone, significant changing of mechanical properties [4], [5]. Relying upon these findings during the analysis of weldability of aluminium and its alloys, it is practical to examine and mention the following: crack sensitivity, porosity, presence of oxide layer, and specialty of heat affected zone [2]. 2.1. Crack sensitivity The most dangerous imperfection (failure) of the welded joint is the crack. There are two types of tendency to crack on welding of aluminium and its alloys, cold- and hot cracking. The incline to hot crack of the joint can be caused by growth of crystals and grains meet but there are liquids between these during the solidification. Additional period of solidification the liquid doesn t get supply. In solidification less and less volume liquid disappears from between of solid grains, arising break of continuity which is enlarged and cracked by forming base material [6]. In terms of hot cracking sensitivity of aluminium alloys the temperature interval between the solidus and liquidus is significant and determinant. In Figure 1. Dependence of hot cracking sensitivity of the weld on the concentration of alloying elements this field the shrinkage is powerful in the course of cooling, while the joint strength is minimal and has reduced deformation capability. 92
Welding of Aluminium Alloys Experience shows if the weld pool contains minimum 15 % eutectic, the hot cracking sensitivity will be reduced to negligible degree. Accordingly the composition of filler material should be chosen in such a way that it contains minimum 3 % Si or minimum 4,5 % Mg. The effects of these components are shown in Figure 2. [2], [7]. In cast aluminium the necessary amount of eutectic easily available from the point of view of avoids hot cracking inclination. Thus, the higher probability of cracking is, the higher the internal stress has that is the higher overheating, product size, specific shrinkage, the amount of thermal expansion coefficient and stiffer grip. Taking into account the previous the cracking risk can be reduced by welding conditions (joint preparation, rigidity of clamping, width of heat affecting) and welding technology (welding process, heat input, welding speed) next to the well-selected filler material composition to the welded alloy (alloy and grain refining element) [1], [2]. 2.2. Susceptibility to porosity The gas solvent ability and its degree of metals and metal alloys are dissimilar furthermore these gases take different effect to the properties of metals and metal alloys depending on the value of gas content. Among the trouble causing gases of welding (oxygen, hydrogen, nitrogen) the hydrogen is dominant in aluminium and its alloys. While the hydrogen solvent ability of molten aluminium forcefully depends on the temperature and is significant, till then in solid state this solution appreciably decreases and at room temperature less than 0,001 cm 3 /100 g metal [5]. Figure 2. The possibility of hydrogen entering to the weld at gas metal arc welding The appearance of hydrogen caused gas inclusion (porosity) is the consequence and reason of this significant difference of melt. Gas inclusions are enhancing the crack sensitivity, resulting embrittlement, significantly decreasing the bending, strongly decreasing the strength (especially yield and fatigue strength) and worsen the corrosion resistance by growing the local internal stress and occurring material discontinuity. The hydrogen can fall into the weld from different places, so thus from the adhesive moisture of the surface of filler material and base material, from contaminants on surfaces (greases, oils), from absorbed gases caused production of material furthermore from applied 93
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Welding of Aluminium Alloys 2. In recent years, developments in electronics and electrical engineering allowed the replacement a part of continuous energy input welding by periodic energy input welding. The application of periodic energy input at fusion welding processes has number of determining advantages in welding of aluminium and its alloys: beside the application of alternating current, the appearance of modern processes of gas tungsten arc welding ensured a * pulse change and modulation of welding, * change of two half-period rate, possibility of arc balance, * control of current up- and down-slope to avoid cold fusion and end crater crack; the primary purpose of periodic energy input of gas (argon) metal arc welding (GMAW) to influence the drop transfer, the possibility of drop frequency planning, which ensures * predictable and controllable linear energy, * avoid the loss of alloying element, * favourable joint properties. 3. Not only fusion welding but pressure welding can successfully be applied to weld aluminium and its alloys. In automotive application the resistance spot welding and friction stir welding are getting more and more attention, because: the short-time welding and change of welding current during time can be successful to break the oxide layer on modern and well-controlled equipment, the friction stir welding which ensures high quality joint without gap and filler material on structural elements. 5. Acknowledgement This research was carried out as part of the TAMOP-4.2.1.B-10/2/KONV-2010-0001 project with support by the European Union, co-financed by the European Social Fund. Authors would like to say special thanks for the financial sponsorship. 6. References [1] [2] [3] [4] [5] [6] [7] [8] Csikós, G.: Alumínium hegesztett szerkezetek gyártása, Hegesztés technika, XVIII, (2007) 3. szám, p.:63-71. Török, I.: Nem-vas anyagok hegesztése, IWE oktatási segédlet. ME. MTT. 1999. p.: 1-109. Török, I.: Alumínium szerkezeti elemek hegesztés technológiájának kidolgozása. NME, MTT, 70/89. Varga, I.: Az alumínium hegesztését befolyásoló néhány anyagtulajdonság, Magyar alumínium, 1984.21. évf. 5. sz. p. 177-183. Kálmán, I.: Az alumíniumhegesztési sajátosságai, Hegesztés technika, XVI, (2005), 3. szám, p.: 13-17. Köves, Z.: Alumínium kézikönyv, Műszaki Könyvkiadó, 1984, Budapest Burai, Z.: Az alumínium és ötvözeteinek hegesztése, Oktatási segédlet, NME, 1983, p.: 1-105. Gáti, J.: Hegesztési zsebkönyv, Műszaki Könyvkiadó, Budapest, 1996. 105
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